The present invention relates to a ceramic resistor comprising a ceramic acting as a resistor material and a method for manufacturing the same.
Ceramic resistors formed of a compound and/or a complex compound containing at least four metallic and/or metalloid elements including Mg and Si have been disclosed in Japanese Unexamined Patent Application Publication Nos. 2-272701, 6-104102, and the like.
These ceramic resistors have resistance against high voltage pulses and high power surges, and also have thermal resistance because their main constituent is ceramic. Thus, they have particular advantages which other resistors do not have.
The methods disclosed in the above-mentioned publications include a step of mixing at least four starting materials (each containing a different element) for 20 hours with a ball mill. If the starting materials were sufficiently mixed, the variation in resistance values of resistors with identical specifications mass-produced from the starting materials would be reduced. However, mixing with a ball mill is not sufficient to reduce the variation in the resistance values.
Accordingly, an object of the present invention is to reduce the variation in resistance values of a ceramic resistor comprising a ceramic acting as a resistor material containing a compound and/or a complex compound containing at least four metallic and/or metalloid elements.
In order to solve the problem described above, the present invention is directed to a method for manufacturing a ceramic resistor containing a ceramic acting as a resistor material. The method includes a step of mixing starting materials including at least four metallic and/or metalloid (for example, Si, but hereinafter Si is regarded as a metal in the description) simple substances, compounds, or complex compounds. A forming step and a firing step follow the mixing step. In the mixing step, means for allowing the starting materials to flow in a whole mixing vessel 1 and means for breaking up aggregates of the starting materials are used.
In order to prepare a ceramic comprising a compound and/or a complex compound containing at least four metallic and/or metalloid elements, a plurality of starting materials are generally used. Those starting materials are generally powders, and they are mixed, subjected to forming, and fired, thus resulting in a ceramic. When the ceramic formed of a compound and/or a complex compound containing at least four metallic and/or metalloid elements is prepared, the above-described mixing step is important. This is because, if the starting materials are, subjected to forming and firing with specific elements (starting materials) aggregated, the resulting ceramic does not exhibit desired characteristics. As described above, when electric characteristics of a ceramic are exploited in, for example, ceramic resistors comprising the ceramic acting as the resistor material, the above described step is particularly important.
It is considered that the known mixing method using a ball mill hardly helps break up aggregates. This is because (1) dry mixing is liable to cause static electricity which induces aggregation; and (2) once the staring materials adhere and aggregate to the inner wall of a mixing vessel or balls formed of ceramic or the like used for a ball mill, it is difficult to break up aggregates of the starting materials. These two reasons (1) and (2) are cited.
Accordingly, the mixing step in the method of the present invention is carried out using means for allowing the staring materials to flow in the whole mixing vessel 1 and means for breaking up aggregates of the starting materials in the mixing vessel 1. The means for allowing the starting materials to flow in the whole mixing vessel 1 is, for example, a first agitating blade 2 shown in FIG. 1 revolving at a relatively low speed (20 to 50 rpm) while rotating at a relatively low speed (1 to 30 rpm). xe2x80x9cRevolvingxe2x80x9d means shifting the rotation axis along a face defined by the rotation direction.
The means for breaking up aggregates in the mixing vessel 1 is, for example, a second agitating blade 3 shown in FIG. 1 rotating at a high speed (2000 to 6000 rpm) to locally agitate the starting materials. Mixing time depends on conditions (rotation speed, revolution speed, viscosities of the materials, and the like), and is generally 10 to 30 minutes. It goes without saying that more time may be spent.
First, using the means for allowing the starting materials to flow in the whole mixing vessel 1, the starting materials are roughly uniformized in the vessel. In addition, aggregates of specific starting materials are sent one after another to the means for breaking up the aggregates, which will be described later. The means for allowing the starting materials to flow may have a function of breaking up the aggregates, but it not necessary. Next, using the means for breaking up the aggregates of the starting materials in the mixing vessel 1, physical shock is given to the aggregates of starting materials to be dispersed. By using these two means, the starting materials can be uniformly dispersed in the whole mixing vessel 1 while the aggregates are being broken up. Hence, an extremely uniform mixture can be prepared.
When a large amount of ceramic resistor material is prepared by subjecting the uniform mixture of the starting materials to forming and firing, the variation in resistance values of the resulting ceramic resistor material can be reduced. The reasons will now be described.
The electrical conduction mechanism of ceramic resistors is base on movement of free electrons and positive holes (carriers) resulting from an incomplete covalent bond between elements. This incomplete covalent bond is formed when compounds each containing an element capable of forming a compound having a different valence are covalent-bonded to each other. If an aggregate of specific starting materials (identical types of metals or metallic compounds) is fired and sintered without being broken, the covalent bond is completely formed in the aggregate, and consequently, carriers hardly move. Thus, the aggregate is liable to prevent electrical conduction. In general, free electrons easily move in comparison with positive holes. Therefore, by forming many regions where free electrons can easily move, the regions can facilitate the electrical conduction. Also, ease of the movement of holes depends on the manner of formation of the holes. Considering all of these factors, it is difficult to reduce the variation in resistance values of the ceramic resistor material unless excellent uniform of the mixture is achieved.
The inventors of the present invention discover a certain degree of correlation between the uniformity and the temperature coefficient of resistance of the ceramic resistor material. The temperature coefficient of resistance (TCR) here represents a rate of change in resistance with temperature (unit: ppm/xc2x0 C.) when resistance is measured at 25 and 125xc2x0 C. in accordance with Section 5. 2. 3 of JIS C 5202. If excellent uniformity is achieved, it leads to a higher temperature coefficient of resistance (xe2x80x9chigherxe2x80x9d here means that a temperature coefficient of resistance changes toward positive values (see FIG. 4)).
In the ceramic resistor formed of a ceramic, acting as a resistor material, containing a compound and/or a complex compound containing at least four metallic and/or metalloid elements, a temperature coefficient of resistance capable of leasing to a reduced variation in resistance values is: 1150 ppm/xc2x0 C. when the specific resistance of the resistor material is 1 kxcexa9xc2x7cm or less; xe2x88x921300 ppm/xc2x0 C. or more when the specific resistance of the resistor material is in the range of 1 to 8 kxcexa9xc2x7cm; xe2x88x921450 ppm/xc2x0 C. or more when the specific resistance of the resistor material is in the range of 8 to 30 kxcexa9xc2x7cm; xe2x88x921530 ppm/xc2x0 C. or more when the specific resistance of the resistor material is in the range of 30 to 70 kxcexa9xc2x7cm; and xe2x88x921620 ppm/xc2x0 C. or more when the specific resistance of the resistor material is 70 kxcexa9xc2x7cm or more. Such temperature coefficients of resistance are an index for determining whether the above-described excellent uniformity is achieve.
Specifically, by satisfying the above-described relationships between the specific resistance and the temperature coefficient of resistance, the variation in resistance values of the ceramic resistor formed of a ceramic acting as a ceramic resistor material containing a compound and/or a complex compound containing at least four metallic and/or metalloid elements can be reduced.
The resistor material contains, for example, Mg and Si. These elements are easily available, and a compound and/or a complex compound containing these elements and other metallic and/or metalloid elements advantageously results in a resistor material capable of providing a wide range of specific resistances.
When Mg and Si are necessary for the resistor material, at least two other metallic and/or metalloid elements of the compound and/or complex compound contained in the resistor material may be at least one selected from the group (first group) consisting of Ca, Zn, Sr, Cd, and Ba; at least one selected from the group (second group) consisting of Sn, Al, Sb, Ga, Pb, Cr, Mn, and Ge; and at least one selected from the group (third group) consisting of Bi, Nb, Ta, V, W, and Mo. The ceramic resistor of the present invention may contain other elements and their compound and/or complex compound as impurities, as long as they do not have serious influence on temperature coefficients of resistance nor do they reduce the effect of reducing the variation in resistance values.
The first group includes alkaline-earth metals. Cd in this group has a harmful effect on the environment. Preferably, at least one selected from Ca, Zn, and Ba is used, from the viewpoint of availability.
The second group includes amphoteric metals. Pb in this group has a harmful effect on the environment. Preferably, at least one selected from Sn, Al, Sb, and Mn is used, from the viewpoint of availability.
The third group includes elements capable of forming compounds having a valence of three or five. Preferably, at least one selected from Bi, V, and W is used, from the viewpoint of availability. By using a ceramic formed of a compound and/or a complex compound containing elements selected from these three groups, Mg, and Si as a resistor material, the resulting ceramic resistor can have resistance against high voltage pulses and high power surges and provide a wide range of specific resistances.